1. Field of the Invention
The present invention is directed to a magnetic resonance tomography apparatus that contains a basic field magnet system and a gradient coil system.
2. Description of the Prior Art
Magnetic resonance tomography is a known technology for acquiring images of the inside the objects of, in particular, the body of a living examination subject. To this end, a magnetic resonance tomography apparatus has a space for the acceptance of the examination subject, what is referred to as an examination space or volume. The examination space is at least partially spatially limited by a surface of the apparatus that surrounds it. The majority of the aforementioned limiting surface is normally formed by a surface belonging to the gradient coil system and another, normally a small portion is formed by a part of an outer envelope of the basic field magnet system. At least in a partial volume sub-region of the examination space, the basic field magnet system generates an optimally uniform, static basic magnetic field on which the gradient coil system superimposes rapidly switched magnetic fields with approximately constant gradients, referred to as gradient fields, in all three spatial directions. Currents whose amplitudes reach several 100 A and that are subject to frequent and fast changes in the current direction with rise and decay rates of several 100 kA/s thereby flow in the gradient coils. These currents are controlled on the basis of pulse sequences and, in the presence basic magnetic field on the order of magnitude of one Tesla, cause oscillations (vibrations) of the gradient coil system due to Lorentz forces.
These oscillations are transmitted via various propagation paths onto the entire surface of the magnetic resonance tomography apparatus. The mechanical oscillations of the various surface regions are transformed into sound oscillations dependent on the surface speed of these surface regions, these sound oscillations ultimately causing sound emissions.
The entire surface of a magnetic resonance tomography apparatus essentially includes the outer envelope of the basic field magnet systemxe2x80x94which forms by far the greatest partxe2x80x94as well as the surface of the gradient coil system, including the devices such as radio frequency antennas mounted at the gradient coil system. Dependent on the measuring location, the envelope of the basic field magnet system is the dominant noise source. This is also true for the examination space, which is essentially limited by the surface of the gradient coil system.
Two transmission paths dominate in the transmission of the oscillations of the gradient coil system onto the envelope of the basic field magnet system. A first transmission path proceeds via a more or less thin intermediate layer between those surfaces of the gradient coil system and the basic field magnet system that directly adjoin one another. This intermediate layer is normally filled by air, which behaves as a transmission medium air. A second transmission path proceeds via a direct mechanical connection of the gradient coil system to the basic field magnetic system, for example due to a press fit of the gradient coil system in a hollow opening of the basic field magnet system.
Developments in the field of magnetic resonance tomography for shortening measuring times and for improving imaging properties involve faster and faster pulse sequences. These cause an increase in the current amplitudes as well as an increase in the current rise and decay rates in the gradient coils. Without counter-measures, this leads to greater noise via more pronounced oscillations due to larger Lorentz forces and a fast change in the effective direction of the Lorentz forces.
German OS 38 33 591 discloses a magnetic resonance tomography apparatus with a tubular gradient coil system which is arranged without supports inside the hollow opening of the basic field magnet system, and which is adjustably carried by a supporting frame that is located outside the basic field magnet system. To this end, the entire gradient coil system is lengthened beyond the longitudinal dimension of the basic field magnet system. The intent is for no mechanical oscillations of the gradient coil system to be transmitted onto the basic field magnet system, and to allow the gradient coil system to be correctly adjustable in the basic magnetic field. The direct mechanical transmission of oscillations via the aforementioned second transmission path is in fact suppressed; however, the aforementioned first transmission path via the intermediate layer is neither damped nor suppressed.
German OS 195 31 216 discloses a magnetic resonance tomography apparatus with a gradient coil system secured to the basic field magnet system via at least one support mount, wherein the support mount is located in the region of an oscillation node of the gradient coil system that is expected during operation. In one embodiment, the support mount includes a damping element. Disadvantageous influences of oscillations of the gradient coil system, for instance acoustic and structural noises, as well as in the image quality (artifacts) are to be avoided as a result. Although improvements again are achieved for the second transmission path, the first transmission path is again neither damped nor suppressed.
German OS 197 34 138 discloses a magnetic resonance tomography apparatus with a gradient coil system arranged in a vacuum encapsulation for reducing noise. The gradient coil system is carried within the vacuum encapsulation by a number of insulating or individually damping fastening devices that are arranged spaced from each other. The fastening devices are formed either as a rubber-like damping fastening with rigid support mount or as a spring-damping fastening with a supporting flange. The fastenings are connected to the gradient coil system and the rigid support mount or the supporting flange of each fastener is connected to the vacuum encapsulation. A damping of the second transmission path and a suppression of the first transmission path dependent on the quality of the vacuum are thereby achieved. Disadvantages arise, however, due to the complete, separate vacuum encapsulation of the gradient coil system. Accessibility to the overall gradient coil system as well as to devices secured to the inside wall of the gradient coil system, for example radio frequency antennas, is significantly impaired due to the vacuum encapsulation, thereby making maintenance and repair more difficult. Further, the vacuum encapsulation of the gradient coil system involves an increase in cost and a reduction of the volume available in the examination space.
In addition to the aforementioned published German applications that disclose structures for effecting damping or suppression of the first and/or second transmission path, German OS 44 32 747 discloses a fundamental reduction of oscillations of the gradient coil system on the basis of an active measure. To that end, a force generator, particularly containing piezoelectric components, is arranged in or at the gradient coil system. These components generate forces that oppose the oscillations of the gradient coil system, so that a deformation of the gradient coil system is essentially prevented. To that end, the piezoelectric components are suitably driven by a voltage applied to them. The introduction or attachment of a number of piezoelectric components into the comparatively spatially extensive gradient coil system, the voltage supply lines thereto, as well as the drive circuitry involve great technical and economic outlay.
An object of the present invention is to provide an economical magnetic resonance tomography apparatus with low noise emission that avoids the aforementioned disadvantages.
This object is inventively achieved in a magnetic resonance tomography apparatus wherein at least a part of a vacuum housing of an evacuatable space is formed by at least one portion (surface region) of the basic field magnet system, preferably a region of the outer envelope thereof, and by at least one portion (surface region) of the gradient coil system. Components thus are utilized for the formation of the evacuatable space for the purpose of noise reduction that are already necessary for the operation of a magnetic resonance tomography apparatus and that often already have the property of vacuum tightness. A closed, separate, expensive vacuum housing is not necessary because regions of the gradient coil system as well as of the basic field magnet form a majority of the vacuum housing.
In an embodiment, the evacuatable space extends at least between those surfaces of the basic field magnetic system and of the gradient coil system facing directly toward one another. As a result, at least the intermediate layer of the initially cited, first transmission path can be contained in a vacuum. The first transmission path for noise-producing oscillations thus can be suppressed.
In another embodiment, the aforementioned vacuum housing is formed by the vacuum housing of a basic field magnet system having superconducting coil arrangement. As a result, for example, a part of the vacuum housing of the superconducting basic field magnet system that is needed anyway is substituted by the gradient coil system. This saves material and thus costs. Moreover, more freedom is gained within the volume in the examination space.
In another embodiment, a further part of the vacuum housing of the evacuatable space is formed by a seal flange. A closed vacuum housing for the evacuatable space between gradient coil system and basic field magnet system already can be realized with the utilization of seal flanges in many apparatuses. This represents a very simple and economical solution. Further, only minimal structural adaptations are necessary given conventional magnetic resonance tomography apparatus to produce the evacuatable space between those surfaces of the basic field magnet system and of the gradient coil system facing directly toward one another. Further, the accessibility of the surface of the gradient coil system facing away from the basic magnet system, and the case for maintenance and repair, are not impaired. The aforementioned surface of the gradient coil systemxe2x80x94to which radio frequency antennas are often securedxe2x80x94remains freely accessible.
In another embodiment, the vacuum housing of the evacuatable space contains a valve device that enables an evacuation of the evacuatable space. The implementation of a seal flange with the aforementioned valve device is especially advantageous. The valve device creates the possibility of evacuating the evacuatable space, for example by pumping out with a vacuum pump, after the mounting of the seal flange has ensued. In conjunction with easily releasable seal flanges, for example for the purpose of maintenance and repair tasks at the complete gradient coil system, a seal flange can be removed and re-mounted after the end of the work and the vacuum can be restored in a simple way.
Further embodiments, in addition to suppressing the first transmission path on the basis of the aforementioned evacuatable space, have at least a damping influence on the second transmission path.
In an embodiment for this purpose, the gradient coil system is connected in a weight-bearing fashion to the basic field magnet system at the location of the dominant natural oscillation node of the gradient coil system. In addition to the suppression of the first transmission path by the vacuum intermediate layer, the noise propagation via the second transmission path is thereby also reduced. The aforementioned German OS 195 31 216 is referenced for a detailed description of the fastening of the gradient coil system at its dominant natural oscillation node.
In a further embodiment, the magnetic resonance tomography apparatus has a carrier for the gradient coil system that enables support of the gradient coil system, preferably at the floor of an installation room, in a manner that is decoupled from the basic field magnet system.
In an embodiment for this purpose, a hollow-cylindrical (tubular) gradient coil system has carrying elements projecting from the lower region of an end face perpendicular to the principal cylinder axis. As a result, a support that is decoupled from the basic field magnet system is possible by lengthening the gradient coil system in that region wherein a transport and support device for an examination subject, for example a patient bed, is normally arranged. Differing from an extension of the complete gradient coil system, the accessibility to the examination space is hardly restricted and the confining volume is not enlarged which is important for acceptance by patients having claustrophobia. The support of the gradient coil system decoupled from the basic field magnet system nearly completely suppresses the second transmission path. Care must thereby be exercised in the fashioning of the evacuatable space, for example on the basis of the attachment of seal flanges, so that a noise producing coupling of the gradient coil system to the basic field magnet system in the sense of a second transmission path does not in turn arise due to the implementation and fastening of the seal flange.
In another embodiment, the basic field magnet system, preferably the outer envelope thereof, and/or a connection between the gradient coil system and the basic field magnet system, contains at least one part of a decoupling mechanism that prevents the propagation of oscillations of the gradient coil system onto the overall outer envelope of the basic field magnet system.
In a particular embodiment, for this purpose, the decoupling mechanism contains a device, preferably embodied as a bellows or an element made of elastic material, that acts in oscillation-decoupling fashion due to its mechanical properties.
In another embodiment for this purpose, the decoupling mechanism contains actuators, preferably embodied as piezoelements, whose spatial size is designed such that they have an oscillation-decoupling effect. The aforementioned German OS 44 32 747 is referenced for a detailed disclosure of the fundamental functioning of piezoelements for oscillation suppression. Compared to the aforementioned publication, however, a number of piezoelements are not arranged over the comparatively spatially extensive gradient coil system; rather, in accordance with the invention piezoelements are arranged in a comparatively small spatial region, for example in the proximity of the connection between the gradient coil system and the basic field magnet system. In this small region, they prevent a transmission of oscillations of the gradient coil system onto the envelope of the basic field magnet system. The economic outlay for this is correspondingly more beneficial and a significant noise-reducing effect is still achieved.